We are studying biochemical events during the transition from quiescence to replicative growth in the cell cycle. These include the synthesis of a protein 5 hr after stimulation of quiescent 3T3 mouse fibroblasts with serum and other mitogens. We are also following changes in pools of water soluble phospholipid derivatives after stimulation. We hope with these studies to find ways of enhancing the differential effects of antifolates and other metabolites on normal and malignant cells. We are also investigating the use of fluorine-18 labeled 5-fluorouridine as a probe of in vivo cellular proliferation in normal and tumor tissue. This approach we hope will provide a means of assessing the effects of various anticancer agents.